The present invention relates to a method of operating a fuel cell so as to generate an electromotive force by chemical reaction between oxygen and a fuel gas (for example, hydrogen), and to a power supply system including the fuel cell as a power supply.
Fuel cells are adapted to generate an electromotive force from generators by chemical reaction between hydrogen representative of a fuel gas and oxygen (air) supplied thereto. In general, each fuel cell has a structure that an electrolyte film (proton conductive film) is sandwiched between gas electrodes, to generate a desired electromotive force by chemical reaction generated therein. Such fuel cells are greatly expected to be applied to electric cars or hybrid vehicles, and further, studies are now being made to improve the fuel cells suitable for use in new applications other than the above-mentioned application thereof mounted on vehicles such as cars, for example, in a portable application where the fuel cells are incorporated in portable equipment, by making use of advantages of the fuel cells in terms of ease for reduction in weight and size as compared with the existing dry cells and chargeable batteries.
In each fuel cell, moisture is generally produced at a cathode electrode by chemical reaction between oxygen and hydrogen. The moisture thus produced may be condensed into a liquid, and the liquid tends to overflow in an oxygen gas passage, to obstruct diffusion of oxygen gas, thereby inhibiting effective reaction. This leads to a problem in reducing the output of each fuel cell.
The kinds of moisture present in a fuel cell may include, in addition to the moisture produced by reaction between hydrogen and oxygen, moisture which has been contained in a humidified hydrogen side gas and has been then permeated on the cathode side through an electrolyte film together with protons, moisture originally contained in the air intake, and moisture initially present in the form of dew condensation in a flow passage. To prevent the reduction in output of the fuel cell, it is required to exhaust the above-described kinds of moisture obstructing an air passage; however, a large scale system is required to detect all of these kinds of moisture and optimally control the exhaust thereof, and therefore, a need exists to realize detection of these kinds of moisture and optimal control of the exhaust thereof by a small-sized and inexpensive system.